Apparatus for forming and washing a fibrous web

ABSTRACT

A layer of fibrous material is formed from a suspension of the fibrous material in a suspension liquid by introducing the suspension into a web-forming space defined between a member impermeable to the suspension liquid and a member that is permeable to the suspension liquid but substantially impermeable to the fibrous material and that is moved along an endless path. The thickness of the web-forming space diminishes from the upstream end to the downstream end thereof, relative to the direction of movement of the permeable member. As the suspension moves through the web-forming space, liquid drains through the liquid permeable member under the influence of pressure developed by the delivery pressure, coupled with the convergence of the space, thereby producing a fibrous layer in the web-forming space. The fibrous layer is washed by supplying liquid under pressure through a permeable element positioned opposite the permeable member downstream from the web-forming space. The volumetric flow rate of liquid is controlled so that the wash liquid displaces the suspension liquid in the web to a depth that is less than the thickness of the layer. The remaining suspension liquid and a small amount of wash liquid are ultimately displaced from the web in a press nip.

United States Patent Friksson et al.

[451 Apr. 22, 1975 APPARATUS FOR FORMING AND WASHING A FIBROUS WEB [75]Inventors: Erik Sture Friksson; Gosta lngemar lngemarsson, both ofKarlstad, Sweden [73] Assignee: Aktiebolaget Karlstads Mekaniska'Werkstad, Karlstad, Sweden [22] Filed: Jan. 21, 1974 [21] Appl. No.:434,935

Related US. Application Data [62] Division of Set. No. 314,164, Dec. 11,1972, Pat. No.

[30] Foreign Application Priority Data Dec. 14, 1971 Sweden 16035/71[52] US. Cl 68/22 R; 68/43; 100/45; 100/75; 100/121 [51] Int. Cl. B05c3/00; B30b 9/20 [58] Field of Search 100/37, 45, 73, 74, 75, 100/121;8/156; 210/402, 403, 404; 162/317, 318, 358; 68/22 R, 43, 45,181 R,158

56 References Cited UNITED STATES PATENTS 3,086,454 4/1963 Asplund100/121 3,220,340 11/1965 Frykhult 100/121 3.342,l24 9/1967 Frykhult100/121 3,616,660 11/1971 lngcrmarsson 68/22 R Primary Examiner-RobertW. Jenkins Assistant E.\'aminerPhilip R. Coe

Attorney, Agent, or Firm-Brumbaugh, Graves, Donohue & Raymond [5 7ABSTRACT A layer of fibrous material is formed from a suspension of thefibrous material in a suspension liquid by introducing the suspensioninto a web-forming space defined between a member impermeable to thesuspension liquid and a member that is permeable to the suspensionliquid but substantially impermeable to the. fibrous material and thatis moved along an endless path. The thickness of the web-forming spacediminishes from the upstream end to the downstream end thereof, relativeto the direction of movement of the permeable member. As the suspensionmoves through the web-forming space, liquid drains through the liquidpermeable member under the influence of pressure developed by thedelivery pressure, coupled with the convergence of the space, therebyproducing a fibrous layer in the web-forming space. The fibrous layer iswashed by supplying liquid under pressure through a permeable elementpositioned opposite the permeable member downstream from the web-formingspace. The volumetric flow rate of liquid is controlled so that the washliquid displaces the suspension liquid in the web to a depth that isless than the thickness of the layer. The remaining suspension liquidand a small amount of wash liquid are ultimately displaced from the webin a press nip.

4 Claims, 2 Drawing Figures APPARATUS FOR FORMING AND WASHING A FIBROUSWEB BACKGROUND OF THE INVENTION This is a division, of application Ser.No. 314,164 filed Dec. ll, l972,"an d now U.S. Pat. No. 3,820,452.

Cellulose is extracted from cellulosic materials, such as wood, bytreating the cellulosic material with certain chemicals in a digester.The chemical digestion process produces a suspension of cellulosicfibers in a suspension liquid that contains various dissolved organicand inorganic substances. After digestion, the cellulosic fibers areseparated from the suspension liquid and washed to remove, aseffectively as possible, the suspension liquid and the dissolved organicand inorganic substances that it contains. It is desirable to save thesuspension liquid rather than discharging it as waste, since some of thechemicals in the suspension liquid can, to advantage, be recovered andreused in the cellulose extraction process. Moreover, some organicsubstances in the suspension are useful as fuel, and it is at leastinappropriate, and it has been made unlawful in many areas, to dischargesuspension liquid into water courses, since the suspension liquidcontains pollutants.

Various processes for recovering useful substances from the suspensionliquid involve evaporation techniques. Accordingly, the quantities ofliquids used in the digestion process should be kept as small aspossible. Similarly, it is desirable that the washing process be carriedout with as little washing liquid as possible, inasmuch as the washingliquid becomes mixed with and dilutes the suspension liquid.

Swedish Pat. No. 318,183 describes apparatus for separating thecellulosic fibrous material produced by a digestion process from thesuspension liquid and for washing the cellulosic fibrous material. Theapparatus described and illustrated in that patent includes a rotatabledrum having a liquid permeable shell and two or more plate or shell-likepressing devices arranged in adjacent relation and defining with theshell a space, the thickness of which diminishes in the direction ofrotation of the drum. A liquid permeable wire moves within the pressingelements, and the suspension of fibrous cellulosic material is suppliedto the space between the wire loop and the drum at a location adjacentthe upstream end of the first pressing element. As the suspension liquidflows through the convergent space defined between the wire and thedrum, the liquid drains inwardly through the shell and outwardly throughthe wire and through the first pressing element, which is perforated orotherwise permeable to liquid, thereby forming a layer of the fibrouscellulosic material in sandwich relation between the shell and wire. Thesuspension liquid is collected both inside the drum and in a casingoutside of the first pressing element and is removed for furtherprocessing. Near the end of the forming space between the drum and thewire, the layers of fibrous material formed on the surface of the drumand on the inside of the wire as suspension liquid is drained arecombined into a single layer which is conveyed between the wire and thedrum into the zone between the second pressing element and the drum.

A second zone, the space between the second pressing element and thedrum, is a washing zone, and washing liquid is supplied under pressurethrough the second pre'ssing element, which is perforated or otherwiseliquid permeable, thereby to displace suspension liquid remaining in thefibrous layer carried between the drum and wire. The fibrous web thenleaves the second pressing element and is conducted between the wire andthe drum through a press nip in which the fibrous web is pressed toremove washing liquid. The web is then separated from the drum and wireand removed from the apparatus.

An important advantage of the apparatus described and illustrated inSwedish Pat. No. 318,183 is that the fibrous web is maintained in acompressed condition during washing in the washing zone, such pressureresulting from the maintenance by the pressing element of a confinedspace of predetermined thickness through which the web moves insandwiched relation between the wire and drum. Accordingly, afterwashing the fibrous web has relatively low liquid content, and bothlumen liquid and volumes of liquid present in the spaces between thefibers of the cellulosic material participate in the displacement ofsuspension liquid by washing liquid. Accordingly, excellent washing isobtained.

SUMMARY OF THE INVENTION The present invention relates to an improvementof the apparatus described and illustrated in Swedish Pat. No. 318,183and similar apparatus. More particularly, the apparatus of that patentis normally operated with washing liquid supplied under high pressure inthe washing zone. Consequently, the washing liquid tends to penetrateentirely through the web along the major part of the length of thewashing zone. As a result, a relatively large quantity of washing liquidis required.

In accordance with the present invention, the rate of supply of washingliquid and the pressure under which it is supplied are controlled sothat the washing liquid does not at any point along the washing zonepass entirely through the layer of fibrous material but displaces thesuspension liquid only to a controlled thickness or depth less than thetotal thickness of the web. Thus, while the apparatus of the presentinvention retains the advantages of the apparatus described in theSwedish Pat. No. 318,183, it provides the additional advantage ofenabling washing to be accomplished with a substantially smaller amountof washing liquid. The apparatus of the present invention also embodiescertain other modifications of the apparatus described in theaforementioned Swedish patent. The present invention also provides amethod for separating fibrous material from a suspension liquid and forwashing the fibrous material, as described more fully hereinafter.

More particularly, the apparatus, according to the invention, includes amember that is permeable to liquids and substantially impermeable to thefibrous material and is movable along an endless path. A memberpositioned opposite the permeable member defines with the permeablemember a layer-forming space, the thickness of which diminishes from theupstream end of the space to the downstream end, relative to thedirection of movement of the permeable member. The suspension of fibrousmaterial is supplied to the upstream end of the forming space. A liquidpermeable element positioned opposite a portion of the permeable memberimmediately downstream from the downstream end of the layer-formingspace defines with the permeable member a washing zone, and wash liquidis supplied under pressure through the permeable element to the washingzone. A press roll located downstream from the downstream end of thewashing zone forms a press nip with the permeable member.

With the apparatus, as described thus far, the suspension liquidsupplied to the layer-forming space is drained through the permeablemember to form a layer, the drainage being a result of forcing thefibrous suspension through the layer-forming space as a result of thediminishing of the thickness of the space and the consequent forcedpressure drainage of liquid through the permeable member. The layer ofmaterial (or fibrous web) leaving the forming space passes through thewash zone where wash liquid is displaced into it and displacessuspension liquid.

In accordance with the invention, the volumetric flow rate of washingliquid supplied to the washing zone is controlled in accordance with thequantitative flow rate of fibrous material through the washing zone.Such control is indirect in the sense that it is obtained by monitoringand controlling other aspects of the operation of the equipment toensure that the washed and pressed layer of fibrous material leaving thepress nip is of substantially uniform dryness and thickness. Uniformityof dryness and thickness of the web upon leaving the press nip isobtained if the linear pressure in the press nip and the thickness ofthe press nip are maintained constant. Under these conditions of uniformdry content and thickness, it is readily possible to compute thequantity of washing liquid to obtain a desired degree of displacement ofthe washing liquid into the fibrous web.

Preferably, the volumetric flow rate of wash liquid is controlled sothat the wash liquid displaces the suspension liquid from the fibrouslayer to a depth at the end of the washing zone of the apparatus that isless than the total thickness of the layer at the end of the washingzone. In other words, the volumetric flow rate of wash liquid isestablished to provide only partial displacement of the suspensionliquid so that the wash liquid does not flow entirely through thefibrous layer at any point in the washing zone. Displacement of all ofthe suspension liquid from the layer, together with a small displacementof washing liquid from the layer is obtained in the press nip, thethickness of the press nip being a value that is less than the depth towhich wash liquid penetrates into the web and displaces suspensionliquid from the layer in the washing zone.

Maintenance of the desired washing effect, as described above, can beobtained by providing in the first instance a constant nip thicknessbetween the press roll and the permeable member that carries the fibrouslayer through the nip. A constant pressure in the nip so that thedryness of the layer as it leaves the nip is constant can be maintainedby either controlling the speed of movement of the permeable member orcontrolling the volumetric rate at which fibrous suspension is suppliedto the forming zone of the apparatus or by a combination of controllingthe speed of movement of the permeable member and the rate of supply offibrous suspension. For example, if fibrous suspension is supplied at aconstant rate to the apparatus, but the permeable member moves at avariable speed, the amount of fibrous material conducted through the nipper unit time is varied in accordance with the speed of movement of thepermeable member. The amount of material per unit time moving throughthe nip directly influences the pressure in the nip.

The volumetric rate of flow of wash liquid in the washing zone iscontrolled in accordance with the volumetric rate of flow of the fibrouslayer through the apparatus. Inasmuch as small variations will occur inthe amount of fibrous material in the layer and thus the nip pressurewill vary by small amounts from time to time, it is preferable that thewash liquid supply be varied in accordance with variations in the nippressure. For example, indirect control of wash liquidsupply inaccordance with nip pressure is conveniently obtainable by monitoringeither the speed of movement of the permeable member or the volumetricrate of flow of fibrous suspension to the layer-forming zone or both.

Thus, the method and apparatus according to the invention, involve usinga minimum amount of wash liquid only partially to displace thesuspension liquid from the layer in the washing zone. Completedisplacement of the washing liquid occurs in the press nip, the pressnip having a thickness no greater than the depth to which wash liquid isdisplaced into the layer in the Washing zone, thereby ensuring that theremaining suspension liquid is displaced from the layer at the pressnip. The invention ensures the maintenance of an appropriate volumetricrate of flow of wash liquid into the fibrous layer and the production ofa washed and pressed layer of fibrous material of substantially uniformthickness, homogenous structure and a high dry content. The displacementof suspension liquid from the layer in the washing zone is obtained witha minimum of pressure difference across the layer so that disruption ofthe fibrous structure is held to a minimum. The layer reaches thewashing zone at the end of the forming zone at a relatively high drycontent, thereby minimizing the quantity of washing liquid required toattain a desired washing effect. A uniform washing effect is assured,because the ratio of the volume of washing liquid supplied to the layerand the dry content of the web as it leaves the press nip are maintainedconstant.

Further features of the invention and the advantages provided by theinvention may be better understood by reference to the followingdescription of an exemplary embodiment, taken in conjunction with thefigures of the accompanying drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a generally schematicside-elevational view of an embodiment of the apparatus of the inventionand depicting a method according to the invention; and

FIG. 2 is a side view of the washing zone and press nip of the apparatusof FIG. 1 illustrating the progress of wash liquid displacement ofsuspension liquid as the layer progresses through the wash zone and thepress nip, the view being on a larger scale than FIG. 1.

DESCRIPTION OF EXEMPLARY EMBODIMENT In the embodiment of the inventionillustrated in the drawings, the permeable member that is moved along anendless path is in the form of a rotatable drum I having a shell 3 thatis permeable-to the suspension and wash liquids but substantiallyimpermeable to the fibrous material. The drum 1 is mounted for rotationin journals 5 and is driven by an appropriate drive system that includesa variable speed drive 7. A shell or casing 9 that extends around alarge portion of the drum surface includes aportion that is impermeableand defines a closed layer-forming space, the thickness of which, as

measured in the radial direction relative to the shell, diminishes fromthe upstream end to its downstream end, relative to the direction ofrotation of the shell (which direction is clockwise in the figures ofthe drawings, as indicated by the arrows).

The 'fibrous suspension to be formed into a layer and washed in theapparatus is delivered at the upstream end of the forming space 11, thesupply apparatus being of a construction such that a substantiallyuniform flow of suspension is supplied along the whole length of theupstream end of the forming zone 11. One or more controllable valves orgates 13 is provided to enable the volumetric flow rate of the fibroussuspension to the forming zone to be controlled.

A perforated or otherwise liquid permeable portion of the shell 9extends from the downstream end of the impermeable portion and defineswith the zone of the drum surface over which it extends a washing zone19. Preferably, thethickness of the washing zone (measured radially ofthe drum) diminishes in the downstream direction and terminates at thedownstream end in a slot or opening 15 of substantially uniformthickness. A casing 23 associated with the permeable portion of theshell forms a chamber for the supply of wash liquid to the washing zone19, and wash liquid is conducted from a supply 25 through one or morevariable flow control valves 27 of the type that enables the volumetricflow rate of wash liquid to the washing zone 19 to be controlled. Thewash liquid passes from the chamber through perforations 21 into theweb.

A rotatable press roll 29 located immediately downstream from thedownstream end of the washing zone 19 defines with the drum surface apress nip 35, the surface or shell 31 of the press roll beingliquidimpermeable. The press roll 19 is mounted in journals 33,preferably in the same machine framework as the journals for the drum 1,so that the thickness of the press nip may be maintained substantiallyconstant during operation. A pressure monitor 37 for continuouslymonitoring the pressure in the press nip 35, such as a strain gauge typeof pressure measuring instrument, is associated with the press rolljournals 33 and produces a signal indicative of the linear pressure inthe press nip 35. Apparatus for removing the web of fibrous materialafter it leaves the press nip is provided, such apparatus including adoctor blade .39 and a combined feeding and shredding screw 41.

The operation of the apparatus of the invention is controlled by acontrol system comprising a controller 43 that is connected by controlsignal conductors 45, 47 and 49 to, respectively, the press rollpressure monitor 37, the speed control device 7 and the controllablevalve or valves 27 associated with the washing liquid supply system. Thespecific type and construction of the components of the control systemand the system as a whole may be selected, as a matter of ordinaryengineering skill, by one skilled in the art from a variety of knownelectrical, hydraulic, pneumatic or mechanical components and systemsthat are well known per se.

The above-described apparatus operates in the following manner. Asuspension of fibrous material is supplied at a constant amount per unitof time, i.e., constant flow rate, under pressure to the upstream end ofthe web-forming zone 17 constituted by the space 11, the valve or valves13 being appropriately set to provide the desired flow rate. The drum 1is rotated at a substantially constant speed of a desired, appropriate Ivalue. The pressure of the fibrous suspension delivered to the formingspace is above the pressure within the drum, and consequently suspensionliquid 53 passes through the liquid permeable shell 3 of the drum and isremoved from within the drum through an outlet 50. A fibrous web 55 isprogressively formed as drainage of liquid through the permeable shelloccurs throughout the web-forming space 17. In particular, asillustrated schematically in FIG. 1 of the drawings, the thickness ofthe fibrous web gradually increases as the web moves with the rotatingdrum through the web-forming space until the thickness of the web isequal to the thickness of the space between the shell 3 and the casing 9and thus fills the space. After the web builds to the thickness equal tothe thickness of the forming space, it is thereafter subjected tomechanical compression as it continues to move with the rotating drumthrough the downstream end portion of the web-forming space and throughthe washing zone 19. Liquid continues to drain from the web due to themechanical pressure, but the pressure is maintained by the convergenceof the casing 9 toward the shell 3 of the drum.

As the fibrous web 55 moves through the washing zone 19 of theapparatus, washing liquid 57 is delivered at a generally constant butcontrolled quantity per unit of time, i.e., volumetric flow rate, underpressure into the chamber 23 and passes through the perforations 21 inthe perforated portion of the casing and flows into the fibrous web. Theflow of washing liquid under pressure into the web displaces suspensionliquid 53 remaining in the fibrous web to a degree that is a function ofthe flow rate of liquid into the web. As illustrated in FIG. 2, suchdisplacement occurs continuously and progressively as the web 55 movesthrough the washing zone, the degree or level of penetration of washingliquid into the web and consequent displacement of suspension liquidfrom the web depicted by the line AA. In other words, above the line AAin FIG. 2, washing liquid has displaced suspension liquid from the web,while below the line AA in FIG. 2, suspension liquid remains in the web,washing liquid having not penetrated into the web below the line AA.

The web passes from the washing zone into the press nip 35 where it ispressed to a thickness substantially less than its thickness as it leftthe washing zone through the opening or slot 15 at the downstream end ofthe washing zone. Preferably, the pressure in the press nip is such asto produce a web that leaves the press nip with a dryness of in therange of from 30 to Inasmuch as the shell 31 of the press roll 29 isliquid impermeable, all drainage occurring in the press nip is throughthe shell 3 of the drum 1. Consequently, the first fraction of theliquid content of the web to drain through the shell 3 is the suspensionliquid 53 that remains in the fibrous web 55 as it leaves the washingzone. Subsequently a part of the washing liquid 57 is also pressed fromthe web and through the permeable shell 3. It is evident from theforegoing, and also from a consideration of FIG. 2 of the drawings thatthe only place in the apparatus where wash liquid actually is drained orpressed from the fibrous web is in the press nip. Preferably the amountof washing liquid removed from the web at the press nip is kept to aminimum, thereby to minimize the amount of washing liquid required inthe processing and thus to limit the amount of liquid ultimately to betreated for recovery of valuable chemicals and to condition it fordischarge from the system.

The maintenance of a flow rate of wash liquid into the washing zone toproduce a desired profile in the extent of displacement of wash liquidto the level approximating the line AA in FIG. 2 of the drawingsrequires coordinating several variables employing the control system ofthe apparatus, such variables including the linear pressure in the pressnip 35, the speed of rotation of the drum 1 and the flow rate of washingliquid 57 into the casing. Preferably, the thickness of the press nip 35and the rate of supply of fibrous suspension 51 are maintainedessentially constant. Appropriate control devices known per se and notshown in the figures of the drawings are employed to maintain a constantpress nip thickness and a constant flow rate of fibrous suspension tothe apparatus.

The pressure monitor 37 associated with the press roll continuouslyproduces signals indicative of the actual value of the linear pressurein the press nip 35, and those signals are conducted to the controller43 through conductors 45. The controller includes a device forgenerating a signal indicative of a known, desired pressure value, whichis hereafter referred to as a set point value. The controller furtherincludes a device or devices for continuously monitoring, and fordetecting any difference between, the actual value of linear pressurederived from the pressure monitor and the set point value. Upondetecting a difference between the actual linear pressure and the setpoint value, the control unit produces a signal indicative of suchdifference, and such signal is processed and conducted as a correctionsignal to the speed control device 7 of the drum 1. The correctionsignal conducted to the speed control device 7 is such that when thelinear pressure drops below the set point value in the controller, thespeed of the drum will be reduced, thereby slowing the movement of thedrum, allowing a thicker web to form for a given supply of fibroussuspension, and ultimately increasing nip pressure. Conversely, if thecorrection signal is indicative of an actual linear pressure in thepress nip above the set point value, the speed of the drum 1 isincreased, with an effect that is the reverse of the one just referredto for a decrease in drum speed.

The controller 43 also produces a control signal that is a function ofthe actual value of the speed of the drum, as monitored by a monitorportion of the speed control element 7, and transmits that signal to theservo-controlled valve or valves 27. The servo-valve adjusts the flowrate of washing liquid to the washing zone of the apparatus inaccordance with the flow rate of fibrous material carried by the drumthrough the washing zone.

The above-described embodiment of the invention is intended to be merelyexemplary, and many variations and modifications of it will be readilyapparent to those skilled in the art without departing from the spiritand scope of the invention. All such variations and modifications areintended to be included within the scope of the invention as defined inthe appended claims.

We claim:

1. ln apparatus for forming a layer of fibrous material from asuspension of the fibrous material in a suspension liquid and forwashing the layer with a wash liquid, the apparatus including a memberpermeable to the liquids and substantially impermeable to the fibrousmate rial, means for moving the permeable member along an endless path,a member positioned opposite the permeable member and defining with thepermeable member a layer-forming space the thickness of which diminishesfrom the upstream end of such space to the downstream end, relative tothe direction of movement of the permeable member, means for supplyingthe suspension of fibrous material to the upstream end of the formingspace, a liquid permeable element positioned opposite a portion of thepermeable member immediately downstream from the downstream end of thelayer-forming space and defining with the permeable member a washingzone, means for supplying washing liquid under pressure through thepermeable element to the washing zone, and a press roll positionedopposite the permeable member and defining therewith a press nipadjacent the downstream end of the washing zone, the improvementcomprising means for monitoring the pressure in the layer of fibrousmaterial at the press nip and producing a nip-pressure signal indicativethereof, means for varying the volumetric rate of flow of fibrousmaterial through the nip and for producing a rate signal indicativethereof, and means for controlling the volumetric flow rate of washingliquid to the washing zone in response to said rate signal.

2. The improvement according to claim 1 wherein the means forcontrolling the supply of wash liquid is responsive to said rate signalto supply wash liquid at a volumetric flow rate such that the washliquid displaces suspension liquid from the fibrous layer to a depth Aadjacent the downstream end of the washing zone less than the totalthickness of the layer adjacent the downstream end of the washing zone,and wherein the thickness B of the nip is less than said depth A.

3. The improvement according to claim 1 wherein the means for varyingthe volumetric flow rate of fibrous material through the nip includesmeans for varying the speed of movement of the permeable member, andwherein the means for supplying the suspension of fibrous materialsupplies the suspension at a substantially constant rate.

4. The improvement according to claim 1 wherein the means forcontrolling the volumetric rate of flow of fibrous material through thenip includes means for varying the rate of supply of fibrous suspensionto the layer-forming space, and wherein the means for moving thepermeable member moves the member at a substantially constant speed.

519 UNITED STATES PATENT OFFICE QERTHMATE GE QGRREUHQN Patent No 3, 878698 Dated April 22 1975 Inventor(s) Erik Sture Eriksson; Gosta IngemarIngemarsson It: is certified that error appears in the above-identifierand that said Letters Patent are hereby corrected as shown be].

Item {75] Inventors: "Erik Sture Frikssonp" should be Erik StureEr-iksson;

fia'gned and geafied Ehas sixteenth D &} 6% September 2975 d patent OW iArrest.-

RUTH C. MASON C. MARSHALL DANN Arresting Officer (ummissium'r of Parentsand Trademarks

1. In apparatus for forming a layer of fibrous material from asuspension of the fibrous material in a suspension liquid and forwashing the layer with a wash liquid, the apparatus including a memberpermeable to the liquids and substantially impermeable to the fibrousmaterial, means for moving the permeable member along an endless path, amember positioned opposite the permeable member and defining with thepermeable member a layer-forming space the thickness of which diminishesfrom the upstream end of such space to the downstream end, relative tothe direction of movement of the permeable member, means for supplyingthe suspension of fibrous material to the upstream end of the formingspace, a liquid permeable element positioned opposite a portion of thepermeable member immediately downstream from the downstream end of thelayer-forming space and defining with the permeable member a washingzone, means for supplying washing liquid under pressure through thepermeable element to the washing zone, and a press roll positionedopposite the permeable member and defining therewith a press nipadjacent the downstream end of the washing zone, the improvementcomprising means for monitoring the pressure in the layer of fibrousmaterial at the press nip and producing a nip-pressure signal indicativethereof, means for varying the volumetric rate of flow of fibrousmaterial through the nip and for producing a rate signal indicativethereof, and means for controlling the volumetric flow rate of washingliquid to the washing zone in response to said rate signal.
 1. Inapparatus for forming a layer of fibrous material from a suspension ofthe fibrous material in a suspension liquid and for washing the layerwith a wash liquid, the apparatus including a member permeable to theliquids and substantially impermeable to the fibrous material, means formoving the permeable member along an endless path, a member positionedopposite the permeable member and defining with the permeable member alayer-forming space the thickness of which diminishes from the upstreamend of such space to the downstream end, relative to the direction ofmovement of the permeable member, means for supplying the suspension offibrous material to the upstream end of the forming space, a liquidpermeable element positioned opposite a portion of the permeable memberimmediately downstream from the downstream end of the layer-formingspace and defining with the permeable member a washing zone, means forsupplying washing liquid under pressure through the permeable element tothe washing zone, and a press roll positioned opposite the permeablemember and defining therewith a press nip adjacent the downstream end ofthe washing zone, the improvement comprising means for monitoring thepressure in the layer of fibrous material at the press nip and producinga nip-pressure signal indicative thereof, means for varying thevolumetric rate of flow of fibrous material through the nip and forproducing a rate signal indicative thereof, and means for controllingthe volumetric flow rate of washing liquid to the washing zone inresponse to said rate signal.
 2. The improvement according to claim 1wherein the means for controlling the supply of wash liquid isresponsive to said rate signal to supply wash liquid at a volumetricflow rate such that the wash liquid displaces suspension liquid from thefibrous layer to a depth A adjaCent the downstream end of the washingzone less than the total thickness of the layer adjacent the downstreamend of the washing zone, and wherein the thickness B of the nip is lessthan said depth A.
 3. The improvement according to claim 1 wherein themeans for varying the volumetric flow rate of fibrous material throughthe nip includes means for varying the speed of movement of thepermeable member, and wherein the means for supplying the suspension offibrous material supplies the suspension at a substantially constantrate.